The determinants of work capacity (i.e., power output) will be examined by measuring power-load curves in single permeabilized rat cardiac myocytes. The role of myosin heavy chain (MHC) will be assessed by examining power-load curves in myocytes with varying mixtures of alpha (a) and beta (b) myosin heavy chain. The applicant expects that these studies on the mixtures will provide insight on the role of force generating or detachment in determining the power output. The contribution (direct effect on the cross-bridge transitions or an indirect effect of thin filament activation) of Ca2+ to power output will be evaluated using varying concentrations of calcium in the experiments or changing the sensitivity of the thin filament system to calcium by extracting TnC or using calcium sensitizing agents. The role of titin on shortening dependence of power output will be assessed by studying myocytes with and without titin, while the effect of the extracellular matrix will be evaluated by using multicellular preparations and comparing them with the single myocyte data. The effect of sarcomere length and interfillament spacing on power output will also be evaluated. The alterations in sarcomere length will also be used to ascertain if attachment rates and detachment rates are the determinants of the power output. Finally, some experiments will be carried out on rat skeletal fibers for the purpose of comparison and validation.